You can re-use a discarded painting, but do not work directly over the old paint. First scrape the ridges of paint down to a flat surface, using your palette knife and sandpaper.
Wipe thoroughly and coat it with white paint. The coating will dry faster and harder if the white lead is cut with copal oil varnish.
A brush can be used, but a palette knife will give a better working surface.
Apply the coating just heavily enough to cover the previous painting, and let it dry thoroughly before starting a new painting
